Numerous types of apparatus for stabilizing a blown film tube are known. Sizing cages which include a cylindrically-shaped shell of small diameter rollers must be positioned above the tube frost line to prevent the tube from sticking to the rollers and creating surface defects. However, this location virtually eliminates any size limiting features of the cages as the tube is already solidified. Internal mandrels over which the tube is physically stretched to a final diameter require an internal heat removal mechanism which makes startup and operation difficult and increasing costs. Contact between the tube and the mandrel also causes surface defects.
Rings of air chambers are described in U.S. Pat. No. 3,976,732 to Herrington and U.S. Pat. No. 4,728,277 to Planeta. In Herrington, a plurality of air rings are positioned around the blown tube and have differing diameters to mechanically define the diameter of the blown tube. The rings form a conical, rather than cylindrical shape which do not provide as stable a film diameter. Planeta discloses a film handling device in which a plurality of stabilizing devices which create axially aligned air rings control the shape of a blown tube. Each device uses two oppositely moving air streams parallel to the tube wall to create a low pressure zone to hold the film in position. This does not produce a sufficiently stable film diameter. U.S. Pat. No. 4,655,988 to Shinmoto et al. discloses a vacuum system. A plurality of air-introducing arms are twisted like a vortex to form a structure in which the internal diameter is physically adjustable like a diaphragm. The air provides a buffer between the arms and the film. However, these complex air-based systems rely on the air to impinge on and flow around the blown tube and do not provide a continuous cushion for the tube.
Size feedback systems measure the diameter of the blown film tube above the frostline and vary the amount of air in the tube interior to control the diameter. These devices monitor the tube diameter with sonar or optical sensors. However, as the tube is formed to its final diameter, this method involves compensating for an error in size which has already occurred. This results in a tube having varying diameter depending on each response of the air control system. Additionally, in all known systems, this is accomplished as part of a sealed tube operation in which the tube is sealed typically at a two roller nip. In these systems, the tube diameter expands or contracts as a result of the air volume and pressure change and measurable diameter changes are required to attain a correcting action. This is not fast or accurate enough to permit the use of these systems with diameter control with open tube systems.
Moreover, these size feedback systems for controlling diameter are inadequate when an internal, imperfect seal or plug is used instead of the two roller nip. The tube diameter responds to very slight changes in internal pressure and volume. None of these pressure control systems can respond quickly and accurately enough to prevent diameter changes due to seal leaks. These systems are therefore unacceptable for use with the imperfect seals of open tube processes. Furthermore, in these methods, process disturbances, such as changes in polymer properties or temperature can result in a larger diameter tube and a slightly lower frost line height.